This invention relates to VideoDisc player apparatus and in particular to the servo electronics for a stylus carrying carriage mechanism.
Certain types of VideoDisc systems utilize disc records wherein information is prerecorded by means of geometric variations in tracks or grooves proximate the surface of the disc. The information is reproduced by means of a signal pickup stylus which engages the track or groove and detects the geometric variations representative of the prerecorded signal. In the capacitance type systems the stylus-record interaction operates to form a time varying capacitance as the geometric variations in a particular track are moved past the stylus by the rotation of the disc, which time varying capacitance forms part of a tuned circuit to amplitude modulate a carrier frequency. The amplitude modulation is thereafter detected and converted to video and audio signals suitable for reproduction on standard receivers. In the pressure sensitive systems, geometric variations in the groove apply a time varying force to the stylus which is mechanically coupled to a pressure sensitive transducer for conversion to electrical signals.
VideoDisc systems of this type typically employ disc records having track or groove densities of 6,000 to 10,000 per inch. As a result of such high groove densities, it is difficult to reliably translate the stylus radially across the disc in accordance with normal play. Therefore the signal pickup stylus is mounted in a carriage assembly driven by motive means for translating the stylus radially across the disc generally synchronously with the rotation of the disc. Because the tracks tend to be slightly eccentric the stylus is mounted within the carriage for limited radial movement of the stylus with respect to the carriage. Such relative movement mechanically biases the stylus mounting arm from its home position and undesirably affects a stylus deflection transducer which provides for stop motion and other special effects. In order to compensate for this condition, the relative stylus position with respect to the carriage assembly is monitored, and the carriage translation is controlled to maintain the stylus mounting arm in a generally unbiased condition with the stylus centered over the track.
One such stylus position monitoring system is disclosed in U.S. Pat. No. 4,280,023, filed July 9, 1979 entitled, "Stylus Position Sensing Apparatus for VideoDisc Player" assigned to the assignee of the present invention. That particular invention senses the stylus position by (a) establishing a capacitance between a first electrode fixed to the carriage and a second electrode mounted in fixed relation to the stylus, (b) measuring the change in capacitance caused by changes in the relative proximity of the first and second electrodes to each other by (c) detecting the relative amplitude of an oscillatory signal coupled from the first to the second electrode, and (d) generating a control signal proportional to such coupled signal. The control signal regulates a DC motor which is mechanically coupled to drive the carriage commensurate with the tracking speed of the stylus.
In order to reduce cost and increase reliability it is desirable to replace the DC carriage drive motor with a stepper motor. However, in the presence of track eccentricity it was found that a stepper motor system tends to enter a hunt mode making steps forward and backward in a single disc revolution. This condition is exacerbated if there is a substantial degree of backlash in the mechanical motor-carriage coupling. Under these conditions the carriage assembly vibrates fore and aft objectionably.